Process for the calcination of a pulverized mineral material

ABSTRACT

Pulverized mineral material is calcined by suspending it in a current of gas constituted by a combustive gas and a fuel whose combustion furnishes the necessary calories for the calcination. To enable solid fuels rich in ash, substantially inert and difficult to condition to be used, the fuel is gasified in a fluidized bed hearth by blowing air from below through the bed whereby a gaseous fuel is disengaged from the bed and flows upwardly with the very finest particles of the solid fuel suspended therein. This is mixed with air and the mineral material to calcine the same.

The present invention relates to the calcination of a pulverized mineralmaterial by suspending the material in a gaseous current constituted bya mixture of a combustive gas and a fuel whose combustion by the gasfurnishes the required calories for the calcining reaction.

In the first industrial installations of this type, oil or natural gaswas used as fuel. However, because of the high price of these fuels,they were replaced by less costly combustible materials. However, inconventional suspension calcination processes, wherein the fuel isinjected into the calcination chamber directly with the material to becalcined, the choice of fuels is limited by certain quality criteriadependent on their ability to burn and their composition (ash and sulfurcontent, etc.). For this reason, many fuels whose price is verycompetitive have up to now not been considered for use in suchcalcination installations, such as fuels high in sulfur content, forexample certain types of coal and coke derived from oil, fuels very richin ash, for example oil shale or bitumens, some substantially inertfuels, fuels which are difficult to condition, for example industrial,urban and domestic waste and laundry sludge, fuels producing relativelyfew calories on burning, and the like.

It is the primary object of this invention to permit the use of suchfuels, alone or in admixture, for the calcination of pulverized mineralmaterials.

The calcination apparatus is capable of burning several fuels of thistype.

Generally, the problem consists of assuring the proper combustion ofthese fuels and to prevent certain noxious components or products of thecombustion from mixing with the calcined material.

The above and other objects are accomplished according to the presentinvention with a process which comprises the steps of gasifyng pieces ofa solid fuel in a fluidized bed hearth by blowing air from below throughthe fluidized bed hearth until the solid fuel has been gasified and isdisengaged from the fluidized bed hearth in an upwardly moving flow ofgaseous fuel entraining the very finest solid fuel particles insuspension therein, and mixing air and the pulverized mineral materialwith said flow of gaseous fuel to burn the gaseous fuel and the solidfuel particles suspended therein until the burned fuel furnishes therequired calories for the calcination. The pieces of solid fuel may havea size up to about 20 mm.

The above and other objects, advantages and features of the inventionwill become more apparent from the following detailed description ofcertain now preferred embodiments thereof, taken in conjunction with thegenerally schematic drawing wherein

FIG. 1 shows a vertical section of a calcination apparatus for carryingout the process according to the present invention;

FIG. 2 is an enlarged transverse section taken perpendicularly to thesection of FIG. 1, showing the fuel gasification device of the apparatusof FIG. 1; and

FIG. 3 is a diagrammatic view of an installation for producing clinkerfrom cement and incorporating the calcination apparatus.

Referring now to the drawing and first to FIGS. 1 and 2, there is shownan apparatus for the calcination of a pulverized mineral material, whichcomprises the combination of fuel gasification device 1, calcinationdevice 2 and vertical connecting conduit 16 between hearth chamber 10 ofthe fuel gasification device and calcination chamber 12 of thecalcination device connected to a lower end thereof.

The fuel gasification device includes hearth chamber 10 defined by anenclosing wall and grate 18 which constitutes the bottom or hearth ofthe chamber. The fuel gasification device also includes means 26comprising duct or chute 25 passing through the enclosing wall of thehearth chamber for delivering pieces of a solid fuel to grate 18 to forma solid fuel bed thereon, and means for blowing air into hearth chamber10 through grate 18 whereby fluidized bed 23 of solid fuel pieces isformed on the grate. The illustrated air blowing means comprises airconducting main 22 with a plurality of branch conduits 21 connected torespective windboxes 20 below and along the length of elongated grate18, control valves 24 being mounted between the branch conduits and thewindboxes to permit the flow of air per surface unit of the grate to bevaried from one end of grate 18 to the opposite end thereof. It is alsopossible to vary the air flow through the grate along its length bychanging the permeability thereof therealong.

The illustrated grate is elongated and comprises at least an end portioninclined in the direction of elongation of the grate and permittingfluidized bed 23 to be formed at a progressively variable depth, chute25 being arranged to deliver the solid fuel at the deepest part of thebed, as shown in FIG. 1. The grate is constituted by the upper course ofan endless chain constituted by hinged plates sufficiently spaced fromeach other to form a grate through which air may be blown upwardly fromwindboxes 20. The endless chain is supported by a pair of rolls 17, 17at least one of which is driven to permit entrainment of the chain in aclockwise direction. In the illustrated embodiment, one of the rolls isarranged higher than the other so that the entire length of the uppercourse is inclined and the ascending upper chain course moves in theupward direction from the deepest part of fluidized bed 23. Screen 27 isplaced above the lower end of the upper chain course to receive thepieces of solid fuel from chute 25 and the solid fuel pieces are heapedon the screen to form a non-fluidized bank of the fuel thereon adjacentto the deepest part of the fluidized bed. Furthermore, as shown in FIG.2, flat supports 21 at each side of, and coplanar with, elongated grate18 support banks 19 of non-fluidized solid fuel pieces heaped on theflat supports whereby fluidized bed 23 on grate 18 is retained betweenbanks 19 to impart to fluidized bed 23 a trapezoidal, upwardly wideningtransverse cross section. The clockwise moving endless chainconstituting grate 18 removes residual ashes from the grate at an upperend thereof remote from the deepest bed part into outlet means 29connected to the hearth chamber and receiving the ashes by gravitytherefrom to remove the ashes. Solid fuel delivery means 26 and the ashremoval means are generally conventional.

Normally, the solid fuel is delivered in pieces having a particle sizeup to 10 or 20 mm and obtained by milling in the case of mineral fuelsand by shredding in the case of fuels consisting of urban or vegetablewaste. These pieces of solid fuel are placed on grate 18 and form afluidized bed hearth by the air blown from windboxes 20 below the gratethrough fluidized bed 23, the air blast being sufficient to assurefluidization of the solid fuel in a highly agitated bed and to gasifythe solid fuel, producing partial combustion and an upwardly moving flowof gaseous fuel disengaged from the fluidized bed hearth and entrainingthe very finest fuel particles in suspension therein. The minimum amountof air blown through the fluidized bed hearth must constitute about 30%to 60% of the stoichiometric air required for the complete combustion ofthe fuel to provide good agitation and corresponding fluidization of thebed of solid fuel and a proper stability of the partial fuel combustion.In this manner, the solid fuel is partially burned and partiallygasified.

Due to the upwardly widening, trapezoidal transverse cross section offluidized solid fuel bed 23, the air may be blown through the bed athigh speeds, for example in excess of 10 meters/second, while the speedof the rising flow of the gaseous fuel and suspended fuel particles iskept relatively low in hearth chamber 10 to reduce the amount of flyingparticles above the bed.

A combustible gas whose temperature may vary between 900° C. and 1100°C. and which is charged with the finest particles of the solid fuel isdischarged from the fluidized bed. Depending on the nature of the fuel,its granulometry and the shape of the hearth chamber, the fuel particlesentrained with the gaseous fuel out of the fluidized bed hearthconstitute about 20% to 50% of the fuel mass.

A large part, i.e. about 50% to 80%, of the residual ashes formed influidized bed 23 are removed by the upwardly moving grate. If the solidfuel has an average or high caloric power, the temperature in thefluidized bed hearth will attain between about 1000° C. and 1200° C.,which is sufficient to cause the residual ashes to agglomerate. Theresultant agglomerates settle in the bed on grate 18 which moves themout of the fluidized bed and discharges them at the upper end of themoving grate into hopper 29. However, if the solid fuel has low caloricpower, the fluidized bed temperature will be too low to permitagglomeration of the ashes at the bottom of the bed. In this case and asshown in FIG. 1, fluidized bed 23 has a depth decreasing progressivelyfrom a first zone whereto the solid fuel is delivered from chute 25 to asecond zone wherefrom the ashes are removed. Valves 24 are so adjustedthat the air blown through the fluidized bed decreases in speed from thefirst to the second zone. In this case, the residual ashes will bedeposited on the upper part of moving grate 18 and thus removed from thebed in a like manner by reducing the air flow per surface unit of grateand the corresponding velocity with which the air is blown therethroughprogressively away from the zone where the solid fuel pieces aredelivered to the grate, i.e. proportionally to the decreasing depth ofthe fluidized bed. Therefore, a fuel high in ash content may always beused without unfavorably influencing the quality of the calcinedmaterial.

If the solid fuel delivered to the hearth chamber is rich in sulfur aswell as in ash, a fraction of the sulfur is affixed to the ash andremoved therewith. However, to enhance the purification of the fuel,particles of a desulfurizing agent may be injected into the fluidizedbed, the desulfurizing agent particles having dimensions between about0.5 and 5 mm. The desulfurizing agent, such as limestone, chalk,dolomite and the like, may be delivered with the solid fuel and willbond part of the sulfur thereto. The particle size of the desulfurizingagent will be so selected that it will be effectively fluidized in theactive portion of the bed and will not be entrained with the flow ofgaseous fuel but will be removed with the ashes.

While the grate has been illustrated as an endless chain, functionallyequivalent grates with moving bars may be used to assure the ascendantdisplacement of slag or ashes from the fluidized bed. Also, the grateneed not be upwardly inclined throughout its length, as shown, but mayhave a horizontal portion at an end zone serving to receive the solidfuel pieces and an upwardly inclined portion at an opposite end zoneserving to discharge residual ashes or slag.

The flow of gaseous fuel and the solid fuel particles suspended thereinare upwardly guided through substantially vertical conduit 16 at a speedof ascension in excess of 20 meters/second to separate calcinationchamber 12. The cross section of the conduit is selected to permit sucha minimum flow velocity to prevent the largest particles suspended inthe gaseous fuel from falling back into the hearth chamber by gravity.The separation of the calcination chamber from the hearth chamber inwhich the fuel is gasified permits the geometry and shape of the hearthchamber to be optimized with a view to minimizing free flying fuelparticles and ashes in the flow of gaseous fuel delivered to thecalcination chamber. The gaseous fuel and the solid fuel particlessuspended therein are completely burned in the separate calcinationchamber.

For this purpose, calcination device 2 includes calcination chamber 12having a vertical axis coaxial with vertical conduit 16. The calcinationchamber is constituted by cylindrical upper part 11 and a lower parthaving an inverted frusto-conical or funnel-shaped wall 13 definingopenings 15 therein. Connecting conduit 16 between hearth chamber 10 offuel gasification device 1 and calcination chamber 12 is connected to alower end thereof. The calcining device further includes annular airdistributing means 28 surrounding the lower calcination chamber part andcommunicating with the calcination chamber through openings 15, inletmeans 30 for delivering the mineral material into the calcinationchamber, and outlet means 32 for the calcined mineral material at anupper end of the calcination chamber. Inlet means 30 comprises a chutedelivering the mineral material to calcination chamber 12 approximatelymidway between the ends thereof. Alternatively and as shown in brokenlines in FIG. 1, the mineral material inlet means may be connected toconnecting conduit 16 to deliver the material with the ascending flow ofgaseous fuel into the calcination chamber. The calcined mineral materialis delivered through outlet means 32 into a cyclone 14, to which theoutlet means is directly connected and which will be further describedhereinafter in connection with FIG. 3.

In this calcination chamber, the air distributed by annular distributor28 through openings 15 and the flow of gaseous fuel are mixed with thepulverized mineral material delivered through inlet means 30 to burn thegaseous fuel and the very fine solid fuel particles suspended thereinuntil the burned fuel furnishes the required calories for thecalcination of the material.

The gaseous fuel current and the pulverized mineral material circulatethrough calcination chamber 12 from the lower to the upper part thereof,the mixture of the gaseous fuel, the air and the mineral material takingplace in the lower calcination chamber part and the combustion productsas well as the calcined mineral material being exhausted at the top ofupper calcination chamber part 11 through outlet duct 32. The exhaustgases are separated from the calcined mineral material in cyclone 14.The flow velocity in cylindrical upper calcination chamber part 11 mustbe sufficient for the entrainment of the calcined mineral materialparticles but must be held within such limits that the very fine solidfuel particles suspended in the gaseous fuel, which have a size of theorder of a few hundred microns, are permitted to dwell in thecalcination chamber long enough to be completely burned. The completecombustion of the gaseous fuel and the fuel fines furnishes the caloriesrequired for the calcination of the pulverized mineral material.

FIG. 3 diagrammatically shows a generally conventional cement plantusing the calcination system of the present invention. The illustratedinstallation includes tubular rotary kiln 40 and two series 42 and 44 ofheat exchange cyclones. The exhaust gases from the kiln pass through thecyclones of series 42 while calcination system 10, 12 is connected tocyclone 14 of series 44 so that the exhaust gases from the calcinationsystem pass through the cyclones of this series. The clinker dischargedfrom the rotary kiln is air-cooled in cooler 46. A fraction of themineral raw material is pre-heated in heat exchanger 42 by means of thekiln exhaust gases passing through its cyclones and the other mineralraw material fraction is pre-heated by means of the exhaust gasesdischarged from calcination chamber 12. The preheated material isdischarged from the last cyclone of each heat exchanger 42 and 44 intocalcination chamber 12 which has respective inlet ducts connecting it tothese cyclones. All of this arrangement is generally conventional andwill, therefore, not be further described herein.

A first fraction of the cooling air at a temperature of the order of200° C. to 300° C. is exhausted from cooler 46 by fan 48 and movedthereby below grate 18 for use in fluidizing the bed of solid fuelpieces and partially burning the fuel in the fluidized bed. A secondfraction of the cooling air at a temperature of the order of 800° C. to850° C. is exhausted from the cooler through duct 50 connected to airdistributor 28 and serves as combustion air for burning the gaseous fueland the solid fuel particles suspended therein. The calcined mineralmaterial moved from the calcination chamber into cyclone 14 is separatedtherein from the exhaust gases coming from the calcination chamber andthe separated calcined mineral material is delivered to rotary kiln 40.

What is claimed is:
 1. A process for the calcination of a pulverizedmineral material, which comprises the steps of(a) gasifying pieces of asolid fuel in a fluidized bed hearth wherein the fluidized bed ismaintained at a depth decreasing progressively from a first zone wheretothe solid fuel is delivered to a second zone wherefrom residual ashesare removed by blowing air from below through the fluidized bed hearthand the air is blown through the fluidized bed at a speed decreasingfrom the first to the second zone until the solid fueld has beengasified and is disengaged from the fluidized bed hearth in an upwardlymoving flow of gaseous fuel entraining the very finest solid fuelparticles in suspension therein, (b) selecting the granulometry of thesolid fuel particles so that the particles entrained with the gaseousfuel out of the fluidized bed hearth constitute about 20% to 50% of thefuel mass, (c) guiding the flow of gaseous fuel and the fuel particlessuspended therein upwardly through a substantially vertical conduit at aspeed of ascension in excess of 20 meters/second to a separatecalcination chamber, and burning the gaseous fuel and the fuel particlesin the separate calcination chamber, and (d) mixing air and thepulverized mineral material with said flow of gaseous fuel to burn thegaseous fuel and the solid fuel particles suspended therein until theburned fuel furnishes the required calories for the calcination.
 2. Thecalcination process of claim 1, wherein the air blown through thefluidized bed hearth constitutes about 30% to 60% of the stoichiometricair required for the complete combustion of the fuel.
 3. The calcinationprocess of claim 1, wherein the temperature in the fluidized bed hearthis maintained between about 1000° C. and 1200° C., and residual ashesare removed from the fluidized bed hearth in the form of agglomeratesdeposited at the bottom of the bed.
 4. The calcination process of claim1, wherein the fluidized bed is laterally retained to form atrapezoidal, upwardly widening cross section.
 5. The calcination processof claim 1, further comprising the step of injecting particles of adesulfurizing agent into the fluidized bed, the desulfurizing agentparticles having dimensions between about 0.5 and 5 mm.
 6. Thecalcination process of claim 1, further comprising the steps ofair-cooling the mineral material, and using a first fraction of thecooling air at a temperature of the order of 200° C. to 300° C. forfluidizing the bed and partially burning the fuel in the fluidized bed,and a second fraction of the cooling air at a temperature of the orderof 800° C. to 850° C. for burning the gaseous fuel and the fuelparticles suspended therein.